1. Field of the Invention
The present invention relates to a method and apparatus for determining the pressure differential between two spaced locations in a fluid-filled biological vessel.
2. Description of Related Art
In recent years there has been an increased desire to obtain in vivo measurements of biological fluid pressure. Fluid pressure measurements are becoming increasingly widespread in cardiology, cardiovascular surgery, urology, gynecology, and gastroenterology. Along with the increase in demand for pressure measurements in a variety of medical applications, there has been an increased desire for greater accuracy and fidelity of such fluid pressure measurements. Further, there is an increasing desire to obtain pressure measurements in small, difficult to reach locations increasing the demand for devices of decreased size and increased maneuverability.
The most common type of device for sensing pressure is the "through the lumen" or "fluid-filled" type of catheter in which the open, distal end of the catheter tube is placed at the location where the fluid pressure is to be determined. Such fluid-filled types of pressure sensing catheters extend from the location where the fluid pressure is to be determined, to an external pressure transducer or manometer outside the patient for recording pressure changes in the fluid-filled lumen. While such fluid-filled pressure catheters do provide a rough approximation of pressure, it is generally recognized that such fluid-filled catheters have a number of associated problems.
One problem with taking pressure measurements through the fluid-filled lumen of such a catheter is that the measurements prevent use of the lumen for other functions. For example, it might be desirable to use the lumen to receive a guidewire for steering and manipulating the catheter, or it might be desirable to use the lumen to introduce contrast medium or drugs. It is usually difficult to use the lumen for more than one function at a time, and providing multiple lumens undesirably increases the overall size of the catheter. Another problem with such catheters is that pressure tracings obtained from fluid-filled catheters are dampened because of the length of the lumen and relatively small diameter of the lumen. Such dampened pressure tracings are only useful in approximating mean pressure. In many applications it is desirable to obtain high fidelity pressure tracing to analyze the rate of pressure change (dP/dT), which is generally not possible with a fluid-filled pressure tracing. An effective barrier to further size reduction of fluid-filled catheters is presented if a fluid-filled pressure lumen having a minimum diameter (e.g., 0.5 mm) is necessary for obtaining useful pressure tracings.
One solution to the shortcomings of fluid-filled pressure catheters is the use of a pressure sensor probe. Such probes use a pressure transducer mounted on the probe which accurately measures fluid pressure in the vicinity of the transducer. For example, the MIKRO-TIP.RTM. probes made by Millar Instruments, Inc. of Houston, Texas (e.g. Model Nos. SPC-450, SPC-330) are effective in providing high fidelity pressure measurements, and may also include a separate lumen for fluid sampling or drug or contrast media injection. Other examples include U.S. Pat. Nos. 3,724,274; 4,274,423; and 4,456,013 (incorporated by reference).
In many medical applications, it is desirable not to obtain absolute pressure readings, but rather to obtain an indication of pressure gradient or differential between two locations in the vessel. For example, in coronary angioplasty, the procedure is usually monitored angiographically, with an intracoronary electrocardiogram, and with fluid pressure measurements taken distal and proximal to the stenosis. Such blood pressure measurements distal and proximal to the lesion are very important in that the transstenotic pressure gradient is an objective and accurate indication of the significance of the stenosis (See e.g., B. Meier, Coronary Angioplasty (1987)).
Conventional methods of obtaining an indication of such pressure gradient are inefficient in several respects. If a fluid-filled lumen type of catheter is used, two separate lumens must be provided--one lumen opening at the first location of interest (e.g. proximal to the lesion) and a second lumen opening at the second location of interest (e.g. distal to the lesion). Providing two separate fluid-filled lumens undesirably increases the size of the catheter, and of course only gives a mean approximation of pressure gradient.
Using a pressure sensor type of catheter is preferable to the fluid-filled lumen approach, but still has several limitations. Such a pressure sensor catheter would include two pressure transducers spaced along the catheter to obtain an indication of absolute fluid pressure at each transducer location. Such a dual pressure sensor catheter requires that both transducers perform accurately and reliably, and the associated leads and transducer mountings give a somewhat complicated catheter assembly. Additionally, the catheter size and profile is driven by the need to accommodate both pressure transducers. Thus, transducer technology limits the size and configuration of such a two pressure sensor catheter. Catheter size and profile is a very important consideration, particularly in coronary angioplasty where the distal end of the catheter must be advanced across a lesion partially occluding the coronary vessel. Therefore, it would be desirable to develop a pressure sensor catheter which could obtain high fidelity pressure differential tracings, while minimizing the catheter size and profile constraints imposed by transducer technology.